Method for producing spherical silicon dioxide granules

Abstract

FIELD: manufacturing ceramic powders.SUBSTANCE: spherical silicon dioxide granules, which can be used to produce functional products, including dental composites. The method for producing spherical silicon dioxide granules, which consists of producing silicon dioxide nanoparticles, preparing a suspension of silicon dioxide nanoparticles, spray drying the suspension, calcining the granules, spray drying is carried out at a suspension feed rate in the range of 4–6 ml / min, a spray temperature in the range of 185–195 °C and compressed air consumption in the range of 476–601 l / h, and calcination of granules is carried out in the temperature range of 840–860 °C.EFFECT: expansion of the range of sizes of silicon dioxide granules and an increase in the quality of the resulting granules, which is achieved by obtaining their spherical shape through the selection of rational spray drying modes and a rational calcination temperature for the granules.1 cl, 1 dwgs, 1 tbls, 7 ex

Description

[0001] The invention relates to the field of manufacturing ceramic powders, namely spherical silicon dioxide granules, which can be used to produce functional products, including dental composites.

[0002] The closest in technical essence and purpose to the proposed invention is the method for producing silicon dioxide granules, selected as a prototype, which includes obtaining silicon dioxide nanoparticles, preparing a suspension of silicon dioxide nanoparticles, spray drying the suspension and calcining the granules [Yang, D.-L.; Sun, Q.; Niu, H.; Wang, R.-L.; Wang, D.; Wang, J.-X. The properties of dental resin composites reinforced with silica colloidal nanoparticle clusters: Effects of heat treatment and filler composition. Compos. Part B Eng. 2020, 186, 107791. https: / / doi.org / 10.1016 / j.compositesb.2020.107791].

[0003] The disadvantage of this method for producing silicon dioxide granules, including the technical challenge, is the small size of the resulting granules, ranging from 0.4 to 8.3 µm (average size 1.6 µm), and the presence of non-spherical granules (irregularly shaped granules and granules with a central hole) in the finished product. Granules smaller than 5 µm are prone to sticking together and forming agglomerates due to van der Waals forces. Such granules are difficult to dose, clump during use, stick to laboratory instruments and glassware, and have poor flow properties. Furthermore, non-spherical granules have a lower bulk density and low packing density.

[0004] The objective of the proposed invention is to improve the processability of a powder consisting of silicon dioxide granules by increasing the average granule size and producing silicon dioxide granules of predominantly spherical shape. Large, dense, spherical particles are packed very efficiently; such powder flows well, does not clump, and is easy to dose. Furthermore, functional coatings are easier and more uniformly applied to spherical granules. Spherical particles of different fractions or compositions mix more homogeneously and exhibit less segregation over time. Spherical particles have a higher bulk density and, consequently, a higher packing density.

[0005] The claimed invention is based on the technical result of expanding the range of sizes of silicon dioxide granules and improving the quality of the resulting granules, which is achieved by obtaining their spherical shape through the selection of rational spray drying modes and a rational calcination temperature for the granules.

[0006] The stated problem is solved, and the claimed technical result is achieved by the fact that in the method for producing spherical silicon dioxide granules, which consists of obtaining silicon dioxide nanoparticles, preparing a suspension of silicon dioxide nanoparticles, spray drying the suspension, calcining the granules, spray drying is carried out at a suspension feed rate in the range of 4-6 ml / min, a spray temperature in the range of 185-195°C and a compressed air flow rate in the range of 476-601 l / h, and calcination of the granules is carried out in the temperature range of 840-860°C.

[0007] The method for producing spherical silicon dioxide granules consists of 4 steps: 1) obtaining spherical silicon dioxide nanoparticles by the sol-gel method; 2) preparing a suspension of silicon dioxide nanoparticles for spraying; 3) spray drying the suspension; 4) calcining the silicon dioxide granules after spraying.

[0008] The first step is to obtain spherical silicon dioxide nanoparticles. Currently, the sol-gel method is one of the most common and accurate methods for producing spherical silicon oxide nanoparticles. In the proposed method, the sol-gel method consisted of hydrolysis and condensation of tetraethoxysilane (TEOS) in an alcohol solvent in the presence of ammonium hydroxide, stirring the resulting solution for 5 hours at a temperature of 23°C, centrifuging the resulting silicon oxide suspension, followed by washing in distilled water and ethyl alcohol, drying and calcining in a muffle furnace at a set temperature of 80°C. The reactions are carried out at low concentrations of ammonia ([NH3] = 0.81 [TEOS]) and distilled water [H2O] = 6.25 [TEOS] in isopropanol. The TEOS content is 0.55 M. The spherical nanoparticles obtained by this method are amorphous silicon dioxide with an average size of 450 nm.

[0009] The second stage involves preparing a suspension for spraying from the obtained spherical silicon dioxide nanoparticles.

[0010] A spray drying suspension typically consists of: the initial particles of the substance to be granulated; a liquid medium in which the initial particles are suspended; surfactants (diflocculants), which prevent the initial particles from sticking together in the suspension and are used to obtain a more stable suspension. They also reduce the viscosity of the raw material, allowing the addition of more solid particles; a binder, which holds the initial particles together during spraying and helps maintain their spherical shape. When preparing the suspension, it is necessary to consider the size and concentration of the initial particles. In the proposed solution, the spray suspension was prepared using distilled water. First, a sample of spherical silica nanopowder obtained by the sol-gel method is loaded into a laboratory beaker. The samples are weighed on a laboratory balance with a weighing accuracy of at least 0.01 g.Next, distilled water (liquid medium) is added to the beaker containing the filler at a ratio of 3:1 to the filler weight. Then, using pipettes, a 4% solution of polyvinyl alcohol (binder) and 0.01% by weight of Dolapix G10 (deflocculant) are added dropwise. The beaker is then placed on a magnetic stirrer at 250 rpm. The speed can be adjusted to prevent the suspension from splashing. The suspension should be continuously stirred for at least 4 hours.

[0011] The third stage involves spray drying the finished suspension. Spray drying is one of the most popular methods for the large-scale production of round granules with a narrow size distribution due to its availability, productivity, and cost-effectiveness, allowing control over the composition and morphology of the resulting particles. Spray drying is a method by which a powder suspension is converted into spherical granules. The dried particles obtained after spray drying are often referred to as microspheres, spherical powders, grains, granulated powders, and granules, which is the term used in the scientific literature. The characteristics of spray-dried granules depend on many parameters. For example, an increase in spray temperature leads to poor particle bonding, overdrying, and the formation of a large number of crumbled particles.Since the ideal granule should be a uniform, solid sphere, excessive drying should be avoided. This process leads to the formation of a hard surface before the droplet achieves its stable spherical shape after atomization, ultimately resulting in a hollow granule. This phenomenon can be explained by the formation of a partial vacuum due to capillary movement of particles from the interior to the surface of the shell. High suspension feed rates can lead to clogging of the spray nozzle, which leads to a halt in the spray process. High compressed air flow rates create significant turbulence and shear forces at the nozzle, breaking the droplets into tiny particles, which in turn can negatively impact granule formation. The original particles may not have enough time and strength to adhere together, resulting in no granules, but only the original powder.At excessive compressed air flow rates, some of the product may escape into the filter without settling in the collection vessel. It is advisable to avoid crushed or irregularly shaped granules, as they disrupt the uniformity of the packaging and can cause defects during subsequent processing of finished products from such granules. Drying temperatures and low slurry and compressed air flow rates that are too low can result in the formation of excessively large droplets that fail to dry and adhere to the cyclone walls, reducing the yield of the finished product. Low temperatures also result in under-dried, damp granules. Furthermore, reducing compressed air flow rates and flow rates reduces process productivity.

[0012] Spraying of silicon dioxide granules is carried out using a spray dryer. The proposed method utilized a Buchi Mini Spray Dryer B-290. A dual-channel nozzle (Ø 0.7 mm) with a Ø 1.4 mm nozzle was used for spraying. Spray drying was performed at a suspension feed rate of 4-6 ml / min, a spray temperature of 185-195°C, and a compressed air flow rate of 476-601 l / h.

[0013] The compressed air flow rate on the selected unit is adjusted using a rotameter float with a 5 mm division value. The selected compressed air flow rate on the rotameter scale was 40-50 mm, which, when converted to the manufacturer's calibration tables, corresponds to 476-601 l / h. A reading of 35 mm on the rotameter corresponded to 414 l / h, and 55 mm to 670 l / h.

[0014] The selected parameters for the suspension feed rate, spray temperature, and compressed air flow rate allowed us to obtain spherical silica granules in the size range of 2 to 19 μm (see examples 2-6 in Table 1). Reducing these parameters resulted in the production of large, irregularly shaped granules (example 1 in Table 1). Increasing the parameters made it impossible to obtain granules; no granules formed (example 7 in Table 1).

[0015] The fourth stage of the proposed method involves calcining the granules. Immediately after spraying, the granules are bound only by polyvinyl alcohol and tend to crumble even with slight mechanical stress.

[0016] To maintain the shape and integrity of the granules and prevent their destruction or chipping, they must be strengthened. The resulting silicon dioxide granules were strengthened by heat treatment at various temperatures. It is known that silicon oxide nanoparticles begin to sinter and transform into glass at temperatures above 1200°C in air, but nanoparticles can also begin to melt at lower temperatures. When selecting the temperatures, we ensured that the temperature did not completely melt the silicon oxide nanoparticles that form the granules, but rather formed so-called sintering necks between them, making the granules resistant to mechanical stress. Furthermore, heat treatment in air above 800°C removes polyvinyl alcohol and residual diflocculant, expanding the range of further uses of the granules and making them biocompatible.The granules were sintered in air using a muffle furnace. The heating rate and holding time for all specified temperatures were 3°C per minute and 1 hour, respectively.

[0017] To assess the correct selection of the sintering temperature, all powders obtained after heat treatment were mechanically sieved through a 63-µm sieve on a vibrating sieve analyzer. The size, shape, and surface morphology of the resulting granules were then examined using a scanning electron microscope. Granules that crumbled, did not retain their shape, or melted after calcination and sieving were rejected.

[0018] The calcination temperature of the granules was experimentally selected to be 840-860°C. Within these limits, strong and stable silicon dioxide granules are formed while maintaining their spherical shape (see examples 3-5 in Table 1).

[0019] Temperatures below 840°C are insufficient to form strong silica granules. Most of the granules disintegrate after calcination and sieving at the specified temperatures (see example 2 in Table 1). At temperatures above 860°C, melting and merging of the nanospheres begins, forming a glassy structure. This disrupts the spherical shape of the particles (see example 6 in Table 1). These granules lose their flowability and original properties.

[0020] Figure 1 shows the microstructure of silicon dioxide granules at various magnifications obtained by the proposed method.

[0021] Examples of the method implementation.

[0022] Below are some examples illustrating the feasibility of the proposed method for producing spherical silicon dioxide granules, Table 1.

[0023]

[0024]

[0025] Example 1.

[0026] The silicon dioxide granules produced by the method described above, using the conditions specified in point 1 of Table 1, were irregularly shaped, with a predominance of granules containing a hole inside (the so-called "donut" shape). The size range of the resulting granules varied from 5 to 27 µm. These granule shapes were due to the low flow rates of the suspension and compressed air. At low flow rates, larger droplets form, which do not have time to dry evenly. The surface of the droplets dries quickly, while the interior remains wet, creating a complex shrinkage and compression mechanism, forming a granule with an internal hole, the so-called "donut" shape. The resulting granules are not suitable for use as fillers, as their irregular shape will limit their flowability, disrupt the uniformity of the packaging, and be a source of defects during subsequent formation of finished products.Further use of granules is not advisable.

[0027] Example 2.

[0028] Silicon dioxide granules produced by the above-described method using the conditions specified in Section 2 of Table 1 were spherical in shape, with a size range from 3 to 19 µm. Increasing the temperature, suspension feed rate, and compressed air flow rate resulted in spherical granules. The resulting granules were then calcined at 830°C and sieved as described above. Microstructure images revealed that 830°C was insufficient to maintain the granules' shape; most of the granules disintegrated into the original nanopowder. Further use of the granules is not advisable.

[0029] Example 3.

[0030] Silicon dioxide granules produced by the above-described method using the conditions specified in Section 3 of Table 1 were spherical in shape with a size range of 3 to 19 µm. The resulting granules were calcined at 840°C and sieved according to the procedure described above. Microstructure images revealed that the granules retained their shape. The resulting granules can be used to produce functional products, including dental composites.

[0031] Example 4

[0032] Silicon dioxide granules produced by the above-described method using the conditions specified in Section 4 of Table 1 were spherical in shape and ranged in size from 2 to 16 µm. Increasing the temperature, suspension feed rate, and compressed air flow rate resulted in spherical granules of a smaller size than those in Example 3. The resulting granules were calcined at 850°C and sieved according to the procedure described above. Microstructure images revealed that the granules also retained their shape. The resulting granules can be used to produce functional products, including dental composites.

[0033] Example 5

[0034] Silicon dioxide granules produced by the above-described method using the conditions specified in Section 5 of Table 1 were spherical in shape and ranged in size from 2 to 13 µm. Increasing the temperature, suspension feed rate, and compressed air flow rate resulted in spherical granules of a smaller size than those in Example 3. The resulting granules were calcined at 850°C and sieved according to the procedure described above. Microstructure images revealed that the granules also retained their shape. The resulting granules can be used to produce functional products, including dental composites.

[0035] Example 6

[0036] The silicon dioxide granules produced by the above-described method, using the conditions specified in Section 6 of Table 1, were predominantly spherical in shape, with sizes ranging from 2 to 13 µm. The resulting granules were calcined at 870°C and sieved as described above. Microstructure images revealed that at the specified temperature, the nanoparticles composing the granules began to melt and merge with each other, disrupting the sphericity of the granules and reducing their specific surface area. These granules are unsuitable for further use.

[0037] Example 7

[0038] Silicon dioxide granules produced by the above-described method using the conditions specified in Item 7 of Table 1 did not form. This effect is due to high temperatures, suspension feed rates, and compressed air consumption. Under these conditions, the droplets formed are too small, while high temperatures and compressed air consumption lead to poor particle binding, resulting in no granules.

[0039] Thus, the claimed set of essential features, reflected in the independent claim of the invention, ensures the achievement of the claimed technical result - an expansion of the range of sizes of silicon dioxide granules and an increase in the quality of the resulting granules, which is achieved by obtaining their spherical shape through the selection of rational spray drying modes and a rational calcination temperature of the granules.

[0040] According to the applicants, the analysis of the claimed technical solution for compliance with the conditions of patentability showed that the features specified in the formula are essential and interconnected with each other to form a stable set of necessary features, unknown at the priority date from the state of the art and sufficient to obtain the required synergistic (super-total) technical result.

[0041] Thus, the above information indicates that the following set of conditions are met when using the declared technical solution:

[0042] - an object embodying the declared technical solution, when implemented, relates to the field of manufacturing ceramic powders, namely spherical silicon dioxide granules, which can be used to obtain functional products, including dental composites;

[0043] - for the claimed object in the form in which it is characterized in the formula, the possibility of its implementation using the means and methods described above in the application or known from the prior art as of the priority date is confirmed;

[0044] - an object that embodies the declared technical solution and, when implemented, is capable of ensuring the achievement of the technical result envisaged by the applicant.

[0045] Consequently, the claimed object meets the patentability criteria of “novelty”, “inventive step” and “industrial applicability” under current legislation.

Claims

A method for producing spherical silicon dioxide granules, which consists of producing silicon dioxide nanoparticles, preparing a suspension of silicon dioxide nanoparticles, spray drying the suspension, and calcining the granules, characterized in that the spray drying is carried out at a suspension feed rate in the range of 4-6 ml / min, a spray temperature in the range of 185-195°C, and a compressed air flow rate in the range of 476-601 l / h, and calcining the granules is carried out in the temperature range of 840-860°C.

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